The disclosure relates generally to power generation systems, and more particularly, to systems for cooling the exhaust gas of power generation systems.
Exhaust gas from power generation systems, for example a simple cycle gas turbine power generation system, often must meet stringent regulatory requirements for the composition of the exhaust gas released into the atmosphere. One of the components typically found in the exhaust gas of a gas turbine power generation system and subject to regulation is nitrogen oxide (i.e., NOx), which includes, for example, nitric oxide and nitrogen dioxide. To remove the NOx from the exhaust gas stream, technology such as selective catalytic reduction (SCR) is often utilized. In an SCR process, ammonia (NH3) or the like reacts with the NOx and produces nitrogen (N2) and water (H2O).
The effectiveness of the SCR process depends in part on the temperature of the exhaust gas that is processed. The temperature of the exhaust gas from a gas turbine power generation system is often higher than about 1100° F. However, SCR catalysts need to operate at less than about 900° F. to maintain effectiveness over a reasonable catalyst lifespan. To this extent, the exhaust gas from a simple cycle gas turbine power generation system is typically cooled prior to SCR.
Large external blower systems have been used to reduce the exhaust gas temperature of a gas turbine power generation system below 900° F. by mixing a cooling gas, such as ambient air, with the exhaust gas. Because of the possibility of catalyst damage due to a failure of an external blower system, a redundant external blower system is typically utilized. These external blower systems include many components, such as blowers, motors, filters, air intake structures, and large ducts, which are expensive, bulky, and add to the operating cost of a gas turbine power generation system. Additionally, the external blower systems and the operation of the gas turbine power generation system are not inherently coupled, thus increasing the probability of SCR catalyst damage due to excess temperature during various modes of gas turbine operation. To prevent SCR catalyst damage due to excess temperature (e.g., if the external blower system(s) fail or cannot sufficiently cool the exhaust gas), the gas turbine may need to be shut down until the temperature issue can be rectified.
Utility power producers use combined cycle (CC) power generation systems because of their inherent high efficiencies and installed cost advantage. CC power generation systems typically include a gas turbine, a heat recovery steam generator (HRSG), and a steam turbine. The heat recovery steam generator uses the hot exhaust gas from the gas turbine to create steam, which drives the steam turbine. The combination of a gas turbine and a steam turbine achieves greater efficiency than would be possible independently.
Operational flexibility to meet varying power grid demands at different times of the day is an important consideration in CC power generation systems. The issue becomes more important as intermittent energy sources such as solar and wind are integrated into the power grid. To this extent, CC power generation systems powered by fossil fuels must be capable of increasing/decreasing power output as required to accommodate such intermittent energy sources.
Non-steady state emissions from a CC power generation system (e.g., during start-up) are generally closely scrutinized by regulatory authorities. During start-up, emission control devices employing selective catalytic reduction (SCR) and carbon monoxide (CO) catalysts are not active. To avoid thermal stresses in the steam turbine, the gas turbine has to be held at a lower load to control the HRSG inlet temperature to around 700° F. Since emission are higher at lower gas turbine loads and the emission control devices are not yet active, emissions during start-up can be an order of magnitude higher than those at steady state operation. Further, operating gas turbines at lower loads for a considerable amount of time also reduces the power provided to the power grid during the crucial start-up period.
Large increases in the electrical power demand placed upon an electrical power distribution grid will tend to reduce the electrical operational frequency of the grid, causing an “under-frequency” grid event. For example, a heavy or sudden electrical demand may cause a particular power distribution grid having a nominal operational frequency of 50 Hz to momentarily operate at 49 Hz. In conventional electrical power generation systems that utilize one or more heavy-duty industrial gas turbines for supplying electrical power to the grid, the physical speed of each gas turbine supplying power to the grid is synchronized to the electrical frequency of the grid. To this extent, during an under-frequency grid event in which the frequency of the grid decreases, the physical speed of the gas turbines will also decrease. Unfortunately, as the physical speed of a gas turbine decreases with other factors being equal, its power output correspondingly decreases. Consequently, during an under-frequency grid event, a gas turbine will tend to output a lower power. In the past, a common practice in response to a power grid under-frequency grid event involved increasing the firing temperature of the gas turbine to produce more power in an effort to maintain a predetermined level of output power. Unfortunately, such over-firing of the gas turbine may reduce the operational life expectancy of various hot gas path components within the gas turbine.